---
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---

# GitHub importer developer documentation

The GitHub importer offers two different types of importers:

- A sequential importer. Used by the `import:github` Rake task.
- A parallel importer. Used by everything else.

The difference between these two importers is:

- The sequential importer does all the work in a single thread, so it's more suited for debugging purposes or Rake tasks.
- The parallel importer uses Sidekiq.

## Prerequisites

- Sidekiq workers that process the `github_importer` and `github_importer_advance_stage` queues (enabled by default).
- Octokit (used for interacting with the GitHub API).

## Code structure

The importer's codebase is broken up into the following directories:

- `lib/gitlab/github_import`: this directory contains most of the code such as
  the classes used for importing resources.
- `app/workers/gitlab/github_import`: this directory contains the Sidekiq
  workers.
- `app/workers/concerns/gitlab/github_import`: this directory contains a few
  modules reused by the various Sidekiq workers.

## Architecture overview

When a GitHub project is imported, we schedule and execute a job for the
`RepositoryImportWorker` worker as all other importers. However, unlike other
importers, we don't immediately perform the work necessary. Instead work is
divided into separate stages, with each stage consisting out of a set of Sidekiq
jobs that are executed. Between every stage a job is scheduled that periodically
checks if all work of the current stage is completed, advancing the import
process to the next stage when this is the case. The worker handling this is
called `Gitlab::GithubImport::AdvanceStageWorker`.

## Stages

### 1. RepositoryImportWorker

This worker starts the import process by scheduling a job for the
next worker.

### 2. Stage::ImportRepositoryWorker

This worker imports the repository and wiki, scheduling the next stage when
done.

### 3. Stage::ImportBaseDataWorker

This worker imports base data such as labels, milestones, and releases. This
work is done in a single thread because it can be performed fast enough that we
don't need to perform this work in parallel.

### 4. Stage::ImportPullRequestsWorker

This worker imports all pull requests. For every pull request a job for the
`Gitlab::GithubImport::ImportPullRequestWorker` worker is scheduled.

### 5. Stage::ImportCollaboratorsWorker

This worker imports only direct repository collaborators who are not outside collaborators.
For every collaborator, we schedule a job for the `Gitlab::GithubImport::ImportCollaboratorWorker` worker.

NOTE:
This stage is optional (controlled by `Gitlab::GithubImport::Settings`) and is selected by default.

### 6. Stage::ImportPullRequestsMergedByWorker

This worker imports the pull requests' _merged-by_ user information. The
[_List pull requests_](https://docs.github.com/en/rest/pulls#list-pull-requests)
API doesn't provide this information. Therefore, this stage must fetch each merged pull request
individually to import this information. A
`Gitlab::GithubImport::PullRequests::ImportMergedByWorker` job is scheduled for each fetched pull
request.

### 7. Stage::ImportPullRequestsReviewRequestsWorker

This worker imports assigned reviewers of pull requests. For each pull request, this worker:

- Fetches all assigned review requests.
- Schedules a `Gitlab::GithubImport::PullRequests::ImportReviewRequestWorker` job for each fetched review request.

### 8. Stage::ImportPullRequestsReviewsWorker

This worker imports reviews of pull requests. For each pull request, this worker:

- Fetches all the pages of reviews.
- Schedules a `Gitlab::GithubImport::PullRequests::ImportReviewWorker` job for each fetched review.

### 9. Stage::ImportIssuesAndDiffNotesWorker

This worker imports all issues and pull request comments. For every issue, we
schedule a job for the `Gitlab::GithubImport::ImportIssueWorker` worker. For
pull request comments, we instead schedule jobs for the
`Gitlab::GithubImport::DiffNoteImporter` worker.

This worker processes both issues and diff notes in parallel so we don't need to
schedule a separate stage and wait for the previous one to complete.

Issues are imported separately from pull requests because only the "issues" API
includes labels for both issue and pull requests. Importing issues and setting
label links in the same worker removes the need for performing a separate crawl
through the API data, reducing the number of API calls necessary to import a
project.

### 10. Stage::ImportIssueEventsWorker

This worker imports all issues and pull request events. For every event, we
schedule a job for the `Gitlab::GithubImport::ImportIssueEventWorker` worker.

We can import both issues and pull request events by single stage because of a specific aspect of the GitHub API. It looks like that under the hood, issues and pull requests
GitHub are stored in a single table. Therefore, they have globally-unique IDs and so:

- Every pull request is an issue.
- Issues aren't pull requests.

Therefore, both issues and pull requests have a common API for most related things.

NOTE:
This stage is optional and can consume significant extra import time (controlled by `Gitlab::GithubImport::Settings`).

### 11. Stage::ImportNotesWorker

This worker imports regular comments for both issues and pull requests. For
every comment, we schedule a job for the
`Gitlab::GithubImport::ImportNoteWorker` worker.

Regular comments have to be imported at the end because the GitHub API used
returns comments for both issues and pull requests. This means we have to wait
for all issues and pull requests to be imported before we can import regular
comments.

### 12. Stage::ImportAttachmentsWorker

This worker imports note attachments that are linked inside Markdown.
For each entity with Markdown text in the project, we schedule a job of:

- `Gitlab::GithubImport::Importer::Attachments::ReleasesImporter` for every release.
- `Gitlab::GithubImport::Importer::Attachments::NotesImporter` for every note.
- `Gitlab::GithubImport::Importer::Attachments::IssuesImporter` for every issue.
- `Gitlab::GithubImport::Importer::Attachments::MergeRequestsImporter` for every merge request.

Each job:

1. Iterates over all attachment links inside of a specific record.
1. Downloads the attachment.
1. Replaces the old link with a newly-generated link to GitLab.

NOTE:
It's an optional stage that could consume significant extra import time (controlled by `Gitlab::GithubImport::Settings`).

### 13. Stage::ImportProtectedBranchesWorker

This worker imports protected branch rules.
For every rule that exists on GitHub, we schedule a job of
`Gitlab::GithubImport::ImportProtectedBranchWorker`.

Each job compares the branch protection rules from GitHub and GitLab and applies
the strictest of the rules to the branches in GitLab.

### 14. Stage::FinishImportWorker

This worker completes the import process by performing some housekeeping
(such as flushing any caches) and by marking the import as completed.

## Advancing stages

Advancing stages is done in one of two ways:

- Scheduling the worker for the next stage directly.
- Scheduling a job for `Gitlab::GithubImport::AdvanceStageWorker` which will
   advance the stage when all work of the current stage has been completed.

The first approach should only be used by workers that perform all their work in
a single thread, while `AdvanceStageWorker` should be used for everything else.

When you schedule a job, `AdvanceStageWorker`
is given a project ID, a list of Redis keys, and the name of the next
stage. The Redis keys (produced by `Gitlab::JobWaiter`) are used to check if the
running stage has been completed or not. If the stage has not yet been
completed `AdvanceStageWorker` reschedules itself. After a stage finishes
`AdvanceStageworker` refreshes the import JID (more on this below) and
schedule the worker of the next stage.

To reduce the number of `AdvanceStageWorker` jobs scheduled this worker
briefly waits for jobs to complete before deciding what the next action should
be. For small projects, this may slow down the import process a bit, but it
also reduces pressure on the system as a whole.

## Refreshing import job IDs

GitLab includes a worker called `Gitlab::Import::StuckProjectImportJobsWorker`
that periodically runs and marks project imports as failed if they have been
running for more than 24 hours. For GitHub projects, this poses a bit of a
problem: importing large projects could take several hours depending on how
often we hit the GitHub rate limit (more on this below), but we don't want
`Gitlab::Import::StuckProjectImportJobsWorker` to mark our import as failed because of this.

To prevent this from happening we periodically refresh the expiration time of
the import process. This works by storing the JID of the import job in the
database, then refreshing this JID TTL at various stages throughout the import
process. This is done by calling `ProjectImportState#refresh_jid_expiration`. By
refreshing this TTL we can ensure our import does not get marked as failed so
long we're still performing work.

## GitHub rate limit

GitHub has a rate limit of 5,000 API calls per hour. The number of requests
necessary to import a project is largely dominated by the number of unique users
involved in a project (for example, issue authors), because we need the email address of users to map
them to GitLab users. Other data such as issue pages and comments typically only requires a few dozen requests to import.

We handle the rate limit by doing the following:

1. After we hit the rate limit, we either:
   - Automatically reschedule jobs in such a way that they are not executed until the rate limit has been reset.
   - Move onto another GitHub access token if multiple GitHub access tokens were passed to the API.
1. We cache the mapping of GitHub users to GitLab users in Redis.

More information on user caching can be found below.

## Caching user lookups

When mapping GitHub users to GitLab users we need to (in the worst case)
perform:

1. One API call to get the user's Email address.
1. Two database queries to see if a corresponding GitLab user exists. One query
   tries to find the user based on the GitHub user ID, while the second query
   is used to find the user using their GitHub Email address.

To avoid mismatching users, the search by GitHub user ID is not done when importing from GitHub
Enterprise.

Because this process is quite expensive we cache the result of these lookups in
Redis. For every user looked up we store five keys:

- A Redis key mapping GitHub usernames to their Email addresses.
- A Redis key mapping a GitHub Email addresses to a GitLab user ID.
- A Redis key mapping a GitHub user ID to GitLab user ID.
- A Redis key mapping a GitHub username to an ETAG header.
- A Redis key indicating whether an email lookup has been done for a project.

We cache two types of lookups:

- A positive lookup, meaning we found a GitLab user ID.
- A negative lookup, meaning we didn't find a GitLab user ID. Caching this
   prevents us from performing the same work for users that we know don't exist
   in our GitLab database.

The expiration time of these keys is 24 hours. When retrieving the cache of a
positive lookup, we refresh the TTL automatically. The TTL of false lookups is
never refreshed.

If a lookup for email returns an empty or negative lookup, a [Conditional Request](https://docs.github.com/en/rest/overview/resources-in-the-rest-api#conditional-requests) is made with a cached ETAG in the header once for every project.
Conditional Requests do not count towards the GitHub API rate limit.

Because of this caching layer, it's possible newly registered GitLab accounts
aren't linked to their corresponding GitHub accounts. This, however, is resolved
after the cached keys expire or if a new project is imported.

The user cache lookup is shared across projects. This means that the greater the number of
projects that are imported, fewer GitHub API calls are needed.

The code for this resides in:

- `lib/gitlab/github_import/user_finder.rb`
- `lib/gitlab/github_import/caching.rb`

## Mapping labels and milestones

To reduce pressure on the database we do not query it when setting labels and
milestones on issues and merge requests. Instead, we cache this data when we
import labels and milestones, then we reuse this cache when assigning them to
issues/merge requests. Similar to the user lookups these cache keys are expired
automatically after 24 hours of not being used.

Unlike the user lookup caches, these label and milestone caches are scoped to the
project that is being imported.

The code for this resides in:

- `lib/gitlab/github_import/label_finder.rb`
- `lib/gitlab/github_import/milestone_finder.rb`
- `lib/gitlab/github_import/caching.rb`

## Logs

> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48512/diffs) in GitLab 13.7.
> - Number of imported objects [introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64256) in GitLab 14.1.
> - `Gitlab::GithubImport::Logger` [introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65968) in GitLab 14.2.
> - `import_source` [renamed](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67726) to `import_type` in GitLab 14.2.

The import progress can be checked in the `logs/importer.log` file. Each relevant import is logged
with `"import_type": "github"` and the `"project_id"`.

The last log entry reports the number of objects fetched and imported:

```json
{
  "message": "GitHub project import finished",
  "duration_s": 347.25,
  "objects_imported": {
    "fetched": {
      "diff_note": 93,
      "issue": 321,
      "note": 794,
      "pull_request": 108,
      "pull_request_merged_by": 92,
      "pull_request_review": 81
    },
    "imported": {
      "diff_note": 93,
      "issue": 321,
      "note": 794,
      "pull_request": 108,
      "pull_request_merged_by": 92,
      "pull_request_review": 81
    }
  },
  "import_source": "github",
  "project_id": 47,
  "import_stage": "Gitlab::GithubImport::Stage::FinishImportWorker"
}
```

## Metrics dashboards

To assess the GitHub importer health, the [GitHub importer dashboard](https://dashboards.gitlab.net/d/importers-github-importer/importers-github-importer)
provides information about the total number of objects fetched vs. imported over time.
